[release] 0.39.0 - GRPC Integration + Event logging rework (#296)

### New Features
- Support grpc and grpc streaming for download_config_specs, see
[docs](https://docs.statsig.com/server/concepts/forward_proxy). New
option to pass in `proxy_configs` and use grpc streaming to update
config values
- Reworked background flushing. New option `retry_queue_size` which
allows you to adjust the memory allocated for handling retries.
Deprecated the option `logging_interval`.






>Included In This Release
>- de5dabb kat-statsig
>   - Log event worker fixes (#295)
>- 5772f30 kat-statsig
>   - Rework log event to use worker + event queue (#282)
>- 372a8e7 kat-statsig
>   - GRPC Integration (#264)

statsig/version.py

@@ -1 +1 @@
-__version__ = '0.38.0'
+__version__ = '0.39.0'